The present invention generally relates to limiter circuits, and more particularly to a limiter circuit having an input versus output characteristic which is non-linear in a region outside a linear operating range thereof.
As will be described later on in the present specification in conjunction with drawings, a conventional limiter circuit has an input versus output characteristic in which the upper and lower limit voltages are fixed. Accordingly, an output voltage of the limiter circuit is limited to the upper limit voltage even when an input voltage is greater than the upper limit voltage, and the output voltage of the limiter circuit is limited to the lower limit voltage even when the input voltage is smaller than the lower limit voltage.
As an example of the use of the limiter circuit, there is the case where the limiter circuit is used in a phase locked loop (PLL) circuit which includes a phase comparator and a voltage controlled oscillator (VCO). The phase comparator compares the phases of an input signal of the PLL circuit and an output signal of the VCO, and an oscillation frequency of the VCO is controlled by an output phase error voltage of the phase comparator. However, when the input signal of the PLL circuit becomes discontinuous (or intermittent) for some reason, for example, the operation of the PLL is disturbed and the PLL carries out an erroneous operation. Hence, the limiter circuit is provided at an input stage of the VCO. The limiter circuit limits the amplitude of the output phase error voltage of the phase comparator to a predetermined amplitude range even when the input signal of the PLL circuit becomes discontinuous, so as to prevent the erroneous operation of the PLL circuit. But since the oscillation frequency of the VCO is controlled from the upper or lower limit frequency after the input signal of the PLL circuit becomes discontinuous and the PLL circuit becomes an open loop, there is a problem in that a long setting time is required for the loop operation to reach the steady state. In other words, in the case where the phase error voltage is greater than the upper limit voltage or is smaller than the lower limit voltage of the limiter circuit, the phase error voltage is limited and fixed to the upper or lower limit voltage, and the phase error voltage will not converge to a voltage within a linear operating range between the upper and lower limit voltages of the limiter circuit.
As another example of the use of the limiter circuit, there is the case where the limiter circuit is used in a horizontal frequency oscillator circuit of a television receiver. The horizontal frequency oscillator circuit includes a frequency-to-voltage converter supplied with an external synchronizing signal, the limiter circuit and a VCO. The frequency-to-voltage converter outputs a voltage dependent on a frequency of the external synchronizing signal supplied thereto, and the limiter circuit limits the amplitude of this voltage before supplying the voltage to the VCO and controls an oscillation frequency of the VCO. Hence, even when a signal dropout and noise exist within the external synchronizing signal, the output voltage of the frequency-to-voltage converter is limited to a predetermined amplitude range and the VCO is prevented from carrying out an erroneous operation. However, an output voltage of the VCO is supplied to a horizontal deflection circuit which operates at a high voltage. For this reason, when the operating range of the VCO is set to a wide range by setting the upper limit voltage of the limiter circuit to a voltage in a vicinity of a maximum tolerable voltage of the horizontal deflection circuit, the horizontal deflection circuit operates at the fixed voltage in the vicinity of the maximum tolerable voltage when the input voltage of the limiter circuit is greater than the upper limit voltage, and the load on the horizontal deflection circuit is large. On the other hand, when the upper limit voltage of the limiter circuit is set to a voltage smaller than the maximum tolerable voltage of the horizontal deflection circuit, the load on the horizontal deflection circuit is reduced but the operating range of the VCO becomes narrow.